Shared Control of Thermistor and Dual Purpose Thermistor Line

ABSTRACT

A battery pack can include a temperature sensor that can provide an output that is indicative of a temperature associated with the battery pack. A battery management unit can directly measure the temperature sensor when the battery pack is by itself or engaged with a tool. A charger can directly read the temperature sensor when the battery pack is engaged with the charger. Thus, the temperature sensor can be shared by the battery pack and the charger. The battery pack can utilize a same terminal that provides access to the temperature sensor to indicate a stop-charge signal. The charger can read the stop-charge signal on the same terminal used to directly access the temperature sensor.

FIELD

The present disclosure relates to rechargeable battery packs for powertools, and more specifically to shared control of the thermistor anddual purpose use of the thermistor line.

BACKGROUND

Rechargeable battery packs may provide a power source for cordless powertools. The battery pack may have a battery with a design voltage and mayprovide power to operate a power tool. The battery itself may consist ofa number of individual battery cells that may be combined within thebattery pack to provide a desired voltage. A lithium-ion battery mayhave a design voltage such as 18, 15, 12, or 9 volts, by way ofnon-limiting example. It may be desired to prevent charging and/ordischarging of the cells in a lithium-ion battery when the temperatureof the cells is above or below threshold values. The battery pack maycontain a temperature sensor, such as a thermistor, to allow thetemperature of the cells to be monitored. It may be desired to have thebattery pack monitor its temperature with the thermistor when thebattery pack is on a shelf or in a tool and to allow the charger todirectly use the thermistor to monitor the temperature of the batterypack during charging.

The battery pack may undergo multiple charging operations at variouscharge current levels. To implement the charging at various chargecurrent levels, a signal can be used to indicate when charging at onecharge current level should cease. It may be desired to utilize aterminal on the battery pack to signal charge current reduction and/ortermination to the charger while also utilizing this terminal to allowtemperature measurement by the charger during the charging operation.

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

A rechargeable battery system according to the present disclosureincludes a battery pack operable to deliver a discharge current and toreceive a charge current and a charger operable to engage with thebattery pack and supply a charge current to the battery pack. Thebattery pack can include at least one battery cell, a first pair ofterminals, a temperature sensor extending between the terminals, and abattery management unit adapted to communicate with the temperaturesensor. The battery management unit can also be operable to determine atemperature associated with the battery pack using the temperaturesensor. The charger can include a power supply circuit, a second pair ofterminals that engage with the first pair of terminals, and a chargercontrol module adapted to communicate with the temperature sensorthrough at least one of the first pair of terminals. The charger controlmodule can be operable to directly determine the temperature associatedwith the battery pack using the temperature sensor. The batterymanagement unit determines the temperature associated with the batterypack using the temperature sensor when the battery pack is disengagedfrom the charger. The charger control module determines the temperatureassociated with the battery pack using the temperature sensor when thebattery pack is engaged with the charger. This arrangementadvantageously allows the temperature sensor in the battery pack to beshared by both the battery management unit in the battery pack and bythe charger control module in the charger. The sharing does not occur atthe same time and, rather, the control of the temperature sensor isswitched between the battery management unit and the charger controlmodule.

A battery charger operable to charge a battery pack having at least onebattery cell and a temperature sensor operable to indicate a temperatureassociated with the battery pack can include a power supply circuitoperable to supply a charge current. The charger can also include acharger control module adapted to communicate with the battery packthrough a first terminal on the battery pack. The charger control moduleis operable to directly determine the temperature associated with thebattery pack using the temperature sensor during a charging operationthrough the first terminal. The charger control module is also operableto determine a charge termination signal from the battery pack throughthe first terminal. Thus, the charger can use the same first terminal toboth directly determine the temperature associated with the battery packusing the temperature sensor and also to determine a charge terminationsignal from the battery pack.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a drawing depicting a system of power tools, including abattery pack, power tools, and a charger;

FIG. 2 is a block diagram of an exemplary configuration for a batterypack operably coupled to a battery charger;

FIG. 3 is a block diagram of an exemplary configuration for a batterypack operably coupled to a power tool;

FIG. 4 is a flowchart illustrating the steps undertaken by the batterypack to determine its temperature and to release control of itsthermistor;

FIG. 5 is a flowchart illustrating the charging operation of the batterypack with the charger; and

FIG. 6 is a representation of the voltage level at a terminal of thebattery pack and the operating conditions associated with same.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings. Corresponding reference numerals indicatecorresponding parts throughout the several views of the drawings.

The present disclosure can relate to a system of power tools of the typethat is generally indicated by reference numeral 10 in FIG. 1. Thesystem of power tools 10 can include, for example, one or more powertools 12, a battery pack 16, and a battery pack charger 18. Each of thepower tools 12 can be any type of power tool, including withoutlimitation drills, drill/drivers, hammer drill/drivers, rotary hammers,screwdrivers, impact drivers, circular saws, jigsaws, reciprocatingsaws, band saws, cutoff tools, cutout tools, shears, sanders, vacuums,lights, routers, adhesive dispensers, concrete vibrators, lasers,staplers, and nailers. In the particular example provided, system ofpower tools 10 includes a first power tool 12 a and a second power tool12 b. For example, first power tool 12 a can be a drill/driver similarto that which is described in U.S. Pat. No. 6,431,389, while secondpower tool 12 b can be a circular saw similar to that which is describedin U.S. Pat. No. 6,996,909. Battery pack 16 can be selectively removablycoupled to first and second power tools 12 a and 12 b to provideelectrical power thereto. Battery pack 16 can also be selectivelyelectrically coupled to charger 18 to charge battery pack 16. It isnoteworthy that the broader aspects of this disclosure are applicable toother types of battery-powered devices.

FIG. 2 illustrates an exemplary configuration of a battery pack 16operably coupled to charger 18. Battery pack 16 is generally comprisedof a plurality of battery cells 20, a battery management unit (BMU)(also known as a battery control unit) 22, and various battery controlcircuits. However, the exemplary configuration is merely provided as acontext for describing the various methods and circuits disclosedherein.

Battery pack 16 may include a plurality of battery cells 20 connected inseries, and/or a plurality of serially connected strings of cells, inwhich the strings are in parallel with one another. For purposes ofdescribing the exemplary embodiments, battery pack 16 may be composed ofcells 20 having lithium-ion cell chemistry. In the context of cordlesspower tools, the nominal voltage rating of battery pack 16 is typicallyat least 18 volts. However, other voltage ratings are contemplated fordifferent applications. In addition, battery pack 16 may be composed ofcells of another lithium-based chemistry, such as lithium metal orlithium polymer, or other chemistry. Furthermore, battery packs 16having cells that need temperature based control and/or over/undervoltage control can also be used and may benefit from the teachings ofthe present disclosure. Although not limited hereto, battery pack 16 ispreferably rechargeable.

BMU 22 is embedded within battery pack 16 and is responsible forprotecting cells 20 and monitoring fault conditions which may develop.In an exemplary embodiment, BMU 22 is implemented in software on adigital microprocessor and can include additional analog batterymonitoring ICs. However, BMU 22 may be embodied in hardware or softwareas a digital microcontroller, a microprocessor or an analog circuit, adigital signal processor, or by one or more digital ICs such asapplication specific integrated circuits (ASICs), for example. Onesuitable BMU 22 includes a Microchip PIC16F616 available from MicrochipTechnology Inc. and one or more ICs, such as Seiko S-8254 Series ICsavailable from Seiko Instruments, Inc. This BMU 22 is readily availableand of a low cost. This BMU 22 can provide two separate and distinctsignals indicative of the voltage of cells 20 relative to two thresholdswithout specifying the voltage of any particular cell 20. Otherexemplary BMUs 22 can include two or more ICs, such as Seiko S-8204BSeries ICs, which can be stacked together to get a higher voltage andwhich may preclude the need for a microprocessor. It should beappreciated that the teachings of the present disclosure can be utilizedby battery packs that do not have a BMU as described herein by usingcustom circuits producing the same output although all of the advantagesmay not be realized.

BMU 22 can include a voltage monitoring circuit 24. In an exemplaryembodiment, voltage monitoring circuit 24 is integral with BMU 22. Inother embodiments, voltage monitoring circuit 24 can be separate fromBMU 22. Voltage monitoring circuit 24 may be configured to senseindividual cell voltage and sense total pack voltage of cells 20.Voltage monitoring circuit 24 provides a signal representing theindividual cell and/or stack voltage that BMU 22 can utilize.Alternatively, BMU 22 may direct voltage monitoring circuit 24 toperiodically measure cell voltage across each cell 20 of battery pack 16and the total battery pack 16 voltage in a sequential manner. A currentaverage cell voltage may be determined by dividing the measured totalvoltage of battery pack 16 by the number of cells 20 in battery pack 16.BMU 22 can output a simple two-state signal indicative of any cell 20being at or above or below a low-voltage threshold, as described below.BMU 22 can also output a separate simple two-state signal indicative ofany cell 20 being at or above or below a high-voltage threshold, asdescribed below. The low and high-voltage thresholds can be set at thefactory when manufacturing BMU 22.

A temperature sensor 26 may be configured to measure the temperature ofcells 20. BMU 22 can include a pull-up resistor that can be selectivelyput in series with temperature sensor 26 and a voltage applied thereto.The voltage can be divided between the pull-up resistor and temperaturesensor 26 such that the voltage of temperature sensor 26 is indicativeof a temperature associated with battery pack 16 and can be determinedby BMU 22. The pull-up resistor can be 10K by example. Temperaturesensor 26 may be implemented with a negative temperature coefficient(NTC) thermistor, as shown, a positive temperature coefficient (PTC)thermistor, temperature sensing integrated circuits, or thermal couplesby way of non-limiting example. BMU 22 normally (by default) provideshigh impedence, such as an open circuit by example, between temperaturesensor 26 and BMU 22. BMU 22 can also provide a short, such as to groundor to the cathode (−polarity) of cells 20, across temperature sensor 26by activating a switch, such as an FET by example.

Referring to FIG. 2, battery pack 16 is selectively coupled to charger18. Charger 18 is generally comprised of a power supply circuit 27 and acharger control module 28. Charger 18 may include a terminal voltagedetection circuit 29 and a watchdog circuit 31. It is envisioned thatother sensing and/or protection circuits may also be incorporated intocharger 18. However, this exemplary configuration is merely provided asa context for describing the various protection methods and circuitsdisclosed herein.

Charger control module 28 is responsible for charging cells 20 andmonitoring any fault condition which may develop. Charger control module28 can also take control of temperature sensor 26 and monitor thetemperature of battery pack 16 when inserted in charger 18. Chargercontrol module 28 can include a pull-up resistor 33 that can beselectively placed in series with temperature sensor 26 and a voltageapplied thereto. Pull-up resistor 33 can be 10K by example. The voltageis divided between pull-up resistor 33 and temperature sensor 26 and canbe determined by voltage detection circuit 29. The voltage oftemperature sensor 26 is thereby indicative of a temperature associatedwith battery pack 16. The measuring of the temperature of battery pack16 by charger 18 is described in more detail below. In an exemplaryembodiment, charger control module 28 is implemented in software on adigital microcontroller. However, charger control module 28 may beembodied in hardware or software as a digital microcontroller, amicroprocessor or an analog circuit, a digital signal processor or byone or more digital ICs such as application specific integrated circuits(ASICs), for example.

Battery pack 16 includes a plurality of terminals or pins 30, 32, 34,36, 38 that are used either when engaged with charger 18 or with powertool 12. First terminal 30 is connected directly to the anode(+polarity) of the most positive cell 20. First terminal 30 can therebybe in communication with the anode. First terminal 30 is utilized whenbattery pack 16 is in power tool 12 and when battery pack 16 is incharger 18. When engaged with power tool 12, first terminal 30 formspart of the discharge path. When battery pack 16 is engaged with charger18, first terminal 30 forms part of the charge path and is engaged withpower supply circuit 27.

Second terminal 32 is a data terminal that is utilized when battery pack16 is engaged with charger 18 and is engaged with charger control module28. Second terminal 32 is used by charger 18 to identify the type ofcells 20 within battery pack 16 and to indicate a pre-charge condition(low-voltage condition) for battery pack 16. Second terminal 32 may alsobe used with power tool 12 to indicate a low-voltage condition which cantrigger a stop to the discharge of cells 20.

Third terminal 34 is a data terminal that is only utilized when batterypack 16 is engaged with charger 18 and is engaged with charger controlmodule 28. Third terminal 34 can be used by BMU 22 to signal chargestep/termination (high-voltage condition) and can also be utilized bycharger 18 to monitor the temperature of battery pack 16.

Fourth terminal 36 only makes contact in charger 18. Fourth terminal 36is the main charge current path for battery pack 16 and engages withpower supply circuit 27 of charger 18. Fourth terminal 36 alsocommunicates with the cathode (−polarity) of cells 20. A fuse 42 can bedisposed between fourth terminal 36 and the cathode of cells 20. Fuse 42can rupture to prevent overcharging of battery pack 16. A switch 44 canbe disposed between fourth terminal 36 and the cathode of cells 20. Theswitch 44 can be an FET and can be normally closed to complete circuitbetween the cathode of cells 20 and fourth terminal 36. BMU 22 canenergize switch 44 to open the charge path and prevent charging ofbattery pack 16.

Fifth terminal 38 is only connected when battery pack 16 engages powertool 12. Fifth terminal 38 is connected directly to the cathode of cells20 and functions as the main discharge current path for battery pack 16in powering power tool 12. Fifth terminal 38 may be a shrouded femaleterminal to prevent accidental shorts. Battery pack 16 can include aswitch 46 disposed between fifth terminal 38 and the cathode of cells20. Switch 46 can be an FET and can be closed to complete circuitbetween the cathode of cells 20 and fifth terminal 38. When it isdesired to open switch 46 to interrupt circuit between fifth terminal 38and the cathode of cells 20, BMU 22 can apply a voltage thereto, such asthrough the line connected to the second terminal 32. It should beappreciated that in some embodiments, switch 46 can be disposed in powertool 12.

Referring now to FIG. 3, a block diagram of battery pack 16 operablycoupled to power tool 12 is shown. Power tool 12 is generally comprisedof a motor 50, an actuation mechanism 52 (such as a trigger assembly byexample), and a tool control module 54. It is envisioned that othersensing and/or protection circuits may be incorporated into power tool12. However, this exemplary configuration is merely provided as acontext for describing the various protection methods and circuitsdisclosed herein.

Tool control module 54 is responsible for allowing motor 50 to drivepower tool 12 along with monitoring fault conditions which may develop.In an exemplary embodiment, tool control module 54 is implemented insoftware on a digital microcontroller. However, tool control module 54may be embodied in hardware or software as a digital microcontroller, amicroprocessor or an analog circuit, a digital signal processor, or byone or more digital ICs such as application specific integrated circuits(ASICs), for example.

First terminal 30 of battery pack 16 communicates with actuationmechanism 52 which in turn communicates with motor 50. First terminal 30forms part of the main discharge current path. Second terminal 32provides data to tool control module 54. As stated above, BMU 22 canprovide a varying signal at second terminal 32 based on the voltage ofcells 20. For example, when any cell 20 is below the low-voltagethreshold value, second terminal 32 can be open such that tool controlmodule 54 sees a high impedance. This is also referred to as secondterminal 32 being de-asserted. When the voltage of every cell 20 exceedsthe low-voltage threshold value, BMU 22 can provide a voltage at secondterminal 32 which can be detected by tool control module 54. This isalso referred to as second terminal 32 being asserted. Tool controlmodule 54 can prevent operation of motor 50 when second terminal 32 isasserted, thereby preventing discharge of cells 20 below the low-voltagethreshold value. Additionally, when second terminal 32 is asserted,switch 46 can be energized, thereby disrupting the circuit between motor50 and the cathode of cells 20. Switch 46 can thereby provide a secondlevel of protection to avoid the discharge of cells 20 below thelow-voltage threshold.

Third and fourth terminals 34, 36 are not utilized when battery pack 16is engaged with power tool 12. Fifth terminal 38 communicates with motor50 and forms part of the main discharge current path for battery pack16.

Battery pack 16 and charger 18 according to the present disclosure sharecontrol of temperature sensor 26. In particular, when battery pack 16 isengaged with power tool 12 or not inserted in charger 18, such as whenon a shelf, BMU 22 controls temperature sensor 26. In contrast, whenbattery pack 16 is engaged with charger 18, charger control module 28controls temperature sensor 26 and is responsible for monitoring thetemperature of battery pack 16 while engaged with charger 18, asdescribed below.

Referring to FIG. 4, operation of BMU 22 and the management of batterypack 16 is shown. It should be appreciated that the description of theoperation of BMU 22 discussed herein pertains to the monitoring of thetemperature of battery pack 16 and, as such, it does not describe allthe functionality of BMU 22. BMU 22 does not monitor the temperature ofbattery pack 16 continuously. Rather, BMU 22 periodically wakes up, suchas every three seconds by example, as indicated in block 60, to check onthe status of battery pack 16. When BMU 22 wakes up, BMU 22 ascertainsif battery pack 16 is engaged with charger 18, as indicated in decisionblock 62. To ascertain if battery pack 16 is engaged with charger 18,BMU 22 looks at the voltage at third terminal 34. In particular, whenbattery pack 16 is either by itself or inserted in power tool 12,temperature sensor 26 should have a relatively low voltage or no voltageacross it as no voltage is being applied to temperature sensor 26through BMU 22 or through third terminal 34. Third terminal 34, asstated above, is only engaged when coupled to charger 18 and does notengage with power tool 12. However, when battery pack 16 is engaged withcharger 18, charger control module 28 has a pull-up bias that raises thevoltage at third terminal 34 up to a particular pull-up voltage level,such as the VCC voltage, which results in third terminal 34 having arelatively high voltage. The pull-up voltage level can be 3 V byexample. As such, BMU 22 can ascertain the voltage at third terminal 34and determine whether battery pack 16 is engaged with charger 18 or not.BMU 22 may take multiple measures of the voltage at third terminal 34 toeliminate the effects of noise.

When BMU 22 determines that battery pack 16 is engaged with charger 18,BMU 22 goes to sleep, as indicated in block 64. BMU 22 will subsequentlywake up, as indicated in block 60, at the prescribed periodic intervaland again check to see if battery pack 16 is engaged with charger 18.This procedure continues until BMU 22 determines that battery pack 16 isnot engaged with charger 18.

When BMU 22 determines that battery pack 16 is not engaged with charger18, BMU 22 determines the temperature of battery pack 16, as indicatedin block 66. To ascertain the temperature, BMU 22 engages the pull-upresistor and applies a voltage across both the pull-up resistor andtemperature sensor 26, which are now in series with one another. BMU 22ascertains the voltage drop across temperature sensor 26 which isdirectly related to the temperature of battery pack 16.

BMU 22 determines if the temperature is within an operating range, asindicated in decision block 68. If the temperature of battery pack 16 iswithin the operational range, BMU 22 goes to sleep, as indicated inblock 64. BMU 22 subsequently wakes up as indicated in block 60 andbegins the process all over again.

When BMU 22 determines that the temperature is not within theoperational range, as determined in decision block 68, BMU 22 willprevent the use of battery pack 16, as indicated in block 70. BMU 22 canprevent operation of battery pack 16 such as by activating switches 44and/or 46 which can prevent communication between fourth and fifthterminals 36, 38 and the cathode of cells 20, respectively. Afterpreventing use of battery pack 16, BMU 22 goes to sleep, as indicated inblock 64, and then periodically will wake up, as indicated in block 60.Upon waking up, BMU 22 again goes through the above described routine.After having previously prevented use of battery pack 16, if BMU 22later determines that the temperature is within the operational range,BMU 22 can change the positions of switches 44 and/or 46, as needed, toagain allow use of battery pack 16. It should be appreciated that whenbattery pack 16 is engaged with power tool 12, BMU 22 may continue toprevent use of battery pack 16 while actuation mechanism 52 of powertool 12 is engaged.

Referring now to FIG. 5, the process by which charger 18 takes controlof temperature sensor 26 from battery pack 16 is shown. The processbegins with battery pack 16 being engaged with charger 18, as indicatedin block 80. When battery pack 16 is engaged with charger 18, chargercontrol module 28 applies the pull-up voltage to pull-up resistor 33which is then in series with temperature sensor 26 through third andfourth terminals 34, 36. BMU 22 normally (default) has a high impedancebetween third and fourth terminals 34, 36 such that charger controlmodule 28 sees temperature sensor 26 between third and fourth terminals34, 36. As a result of applying the pull-up voltage, charger controlmodule 28 can then measure the voltage across temperature sensor 26 anddetermine the temperature of battery pack 16, as indicated in block 82.As stated above, whenever BMU 22 wakes up, BMU 22 will detect the highvoltage at third terminal 34 provided by charger control module 28 andwill go back to sleep, thereby maintaining the high impedance thatallows charger control module 28 to see temperature sensor 26.

After charger 18 determines the temperature of battery pack 16, charger18 determines if the temperature is within the operating range, asindicated in decision block 84. If the temperature is outside theoperating range, charger control module 28 will prevent a chargingoperation, as indicated in block 86. To prevent a charging operation,charger control module 28 can activate watchdog circuit 31, therebyinterrupting the connection between power supply circuit 27 and fourthterminal 36 of battery pack 16. Additionally, charger control module 28can also command power supply circuit 27 to no longer supply chargecurrent to battery pack 16.

After preventing the charging operation from occurring, charger controlmodule 28 will signal an error, as indicated in block 88. The error canbe a generic error that is signaled or can be more specific to the typeof error encountered. For example, the error that is signaled can varybased upon the temperature of battery pack 16 being too high or too low.The charging process then ends.

If the temperature of battery pack 16 is determined to be within theoperating range, as decided in decision block 84, charger control module28 will implement a charging operation to charge battery pack 16, asindicated in block 90. During the charging operation, BMU 22 can monitorthe voltage of cells 20 and provide a signal on second terminal 32 whenthe voltage of any particular cell is below or at or above a low-voltagethreshold. For example, BMU 22 can provide a high impedance at secondterminal 32 when any cell 20 is below the low-voltage threshold. Whenall cells 20 are at or above the low-voltage threshold, BMU 22 can applya voltage to second terminal 32. Charger control module 28 can monitorsecond terminal 32 and implement either a pre-charge operation(relatively low-charge current level) or a fast-charge operation(relatively high-charge current level) to charge battery pack 16.

BMU 22 can also provide a signal at third terminal 34 that can functionas a stop-charge signal. In particular, BMU 22 can monitor the voltageof cells 20 and when any particular cell 20 has a voltage that exceeds ahigh-voltage threshold, BMU 22 can pull third terminal 34 low (such asto ground or to fourth terminal 36 by example). When this occurs,charger control module 28 will no longer see temperature sensor 26 and,rather, will see the ground at third terminal 34. When this occurs,charger control module 28 can cease the charging operation, bycommanding power supply circuit 27 to stop supplying the charge currentand/or engaging watchdog circuit 31.

During the charging operation, charger control module 28 monitors thecharging operation, such as by monitoring third terminal 34, and canascertain if the charging operation is complete, as indicated indecision block 92. If the charging operation is not complete, chargercontrol module 28 continues to determine the temperature of battery pack16, as indicated in block 82, determine if the temperature is within theoperation range, as indicated in block 84, and if so continues to chargebattery pack 16 and determine if the charging operation is complete, asindicated in block 90 and decision block 92. This looping continuesuntil either the temperature is not within range, as indicated indecision block 84, or charger control module 28 ascertains that thecharging operation is complete, as indicated in decision block 92.

When charger control module 28 determines that the charging operation iscomplete, as determined in decision block 92, charger control module 28next determines if it is desired to implement another charge step, asindicated in decision block 94. In particular, for some battery packs16, it may be desired to provide differing charge current levels tocharge cells 20 in a sequence. For example, it may be desired toinitially charge cells 20 with a relatively high-charge current leveluntil one of the cells 20 exceeds the high-voltage threshold. As thevoltage of the cells drops to back below the high-voltage. threshold,charger 18 may implement an intermediate charging operation wherein arelatively intermediate charge current is utilized to again charge cells20 until one of cells 20 has a voltage that exceeds the high-voltagethreshold. A third charging operation can utilize a relativelylow-charge current to again charge cells 20 until one of cells 20 isabove the high-voltage threshold. By way of example, a relativelyhigh-charge current can be 2A, an intermediate-charge current can be 0.9A, and a relatively low-charge current can be 100 mA. In someembodiments, the charging operation may continue to charge cells 20anytime the temperature of battery pack 16 is within the operating rangeand the voltage of cells 20 are below the high-voltage threshold.

Thus, when charger control module 28 ascertains that another charge stepis to be implemented, as indicated in decision block 94, the operationreturns to block 82 to determine the pack temperature and proceedsthrough the above-described process. It should be appreciated thatcharger control module 28 can keep track of the number of charging stepsthat are implemented and control the charging operation accordingly.

When it is determined that no more charge steps are to be implemented,as decided in decision block 94, charger control module 28 can signalthat the charging is complete, as indicated in block 96. The chargingprocess is then ended.

Referring now to FIG. 6, a depiction of exemplary voltages that can beseen across temperature sensor 26 at third terminal 34 relative to theoperating conditions is shown. The voltage at third terminal 34 can varyfrom ground (0V) to the pull-up voltage, such as the VCC voltage. TheVCC voltage can be 3V by example. A voltage at third terminal 34 betweenVCC and the low temperature level is an area 97 where no charging ordischarging can occur. Additionally, when this voltage is detected, thecharger control module 28 can prevent charging and signal an error suchas a bad pack or a low-temperature indicator. A voltage between thelow-temperature level and the high-temperature level is an area 98 thatis considered the normal operating range for both charging and operatingbattery pack 16. Voltage between the high-temperature level and the stoplevel is an area 99 wherein no charging or discharging should occur.Additionally, when this occurs, charger control module 28 can blink abad pack or a high temp signal. There is a narrow band between the stoplevel and the ground level. This narrow band is an area 100 that allowsfor the stop level to account for noise that may be picked up at thirdterminal 34 and does not require a 0V signal to be present. When this isthe case, battery pack 16 can signal to charger 18 to stop providingcharge current if charger 18 is doing so.

Thus, in the present disclosure, battery pack 16 can utilize an internaltemperature sensor 26 to monitor the temperature of battery pack 16.When the temperature is outside of an operational band, the battery pack16 can prevent current flow through cells 20. Battery pack 16 utilizestemperature sensor 26 when battery pack 16 is engaged with power tool 12or is by itself. When battery pack 16 is engaged with charger 18,however, BMU 22 relinquishes access to temperature sensor 26 to charger18. Charger 18 can then read temperature sensor 26 directly anddetermine if conditions are appropriate for charging battery pack 16.Charger control module 28 can directly measure temperature sensor 26 toascertain the temperature of battery pack 16. BMU 22 can provide adefault high impedance between BMU 22 and temperature sensor 26 to allowcharger control module 28 to directly read temperature sensor 26. BMUcan also short (pull low, such as to ground) temperature sensor 26 toprevent direct access to temperature sensor 26 by charger control module28. These two functions can both be accomplished utilizing thirdterminal 34. Thus, control of temperature sensor 26 is shared betweenboth battery pack 16 and charger 18.

Furthermore, according to the present disclosure, third terminal 34 canalso be used to signal a charge-current reduction and/or a terminationto charger 18. In particular, as stated above, when the voltage of anycell 20 in battery pack 16 exceeds the high-voltage threshold, BMU 22takes control of third terminal 34 and sends a signal to charger controlmodule 28 that ceases the charging operation. For example, BMU 22 canshort or pull low (such as to ground) third terminal 34 so that chargercontrol module 28 no longer sees temperature sensor 26 and responds byeither terminating the charging operation or implementing anothercharging step. In this way, third terminal 34 is used for two purposes:(1) temperature measurement by charger 18 during a charging operation;and (2) charge termination based on a signal from BMU 22.

The signals described above on third terminal 34 can both be analog innature instead of digital. The analog nature of the signals makes thefunctionality of third terminal 34 consistent with commerciallyavailable, off-the-shelf, battery management IC's, such as thosediscussed above. By allowing charger 18 to directly measure temperature,BMUs 22 can be utilized that do not have temperature protectionfeatures. This can lower the cost or allow a wider range of BMUs to beutilized. By using the output of a standard BMU 22 to toggle thirdterminal 34, the system can be less expensive to build since it can usea standard part and a single terminal to accomplish both temperature andcharge control.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention. Individual elements or features ofa particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the invention, and all such modificationsare intended to be included within the scope of the invention.

What is claimed is:
 1. A power tool system comprising: at least onepower tool; a battery pack comprising a plurality of battery cells and abattery management unit; and a battery charger configured to charge thebattery pack; wherein the battery pack is coupleable with the at leastone power tool to provide power to the at least one power tool; andwherein the battery pack is operatively coupleable with the batterycharger such that the battery charger can charge the battery pack;wherein the battery management unit is configured to output a firstsignal indicative of a voltage of at least one of the battery cellsrelative to a low-voltage threshold; and wherein the battery managementunit is further configured to output a second signal indicative of avoltage of at least one of the battery cells relative to a high voltagethreshold.
 2. The power tool system according to claim 1, wherein thebattery pack comprises at least one first terminal; and wherein thebattery charger comprises at least one second terminal that engages withthe at least one first terminal when the charger is coupled to thebattery pack.
 3. The power tool system according to claim 1, wherein thefirst signal is a simple two-state signal.
 4. The power tool systemaccording to claim 3, wherein the first signal is indicative of thevoltage of at least one of the battery cells being at, above or below alow-voltage threshold.
 5. The power tool system according to claim 1,wherein the second signal is a simple two-state signal.
 6. The powertool system according to claim 5, wherein the second signal isindicative of a cell of the at least one cells being at, above or belowa high-voltage threshold.
 7. The power tool system according to claim 1,wherein the battery pack includes a plurality of battery cells; whereinthe battery management unit includes a voltage monitoring circuitconfigured to sense individual cell voltages of the plurality of batterycells and a total voltage of the plurality of battery cells.
 8. A powertool system comprising: at least one power tool; a battery packcomprising at least one battery cell and a battery management unit; anda battery charger configured to charge the battery pack; wherein thebattery pack is coupleable with the at least one power tool to providepower to the at least one power tool; and wherein the battery pack isoperatively coupleable with the battery charger such that the batterycharger can charge the battery pack; wherein the battery management unitis configured to determine whether the battery pack is operativelycoupled with the battery charger; wherein the battery management unit isfurther configured such that when the battery management unit determinesthat the battery pack is operatively coupled with the charger, thebattery management unit powers down; wherein the battery management unitis further configured such that when the battery management unitdetermines that the battery pack is not engaged with the charger, thebattery management unit determines a temperature of the battery pack. 9.The power tool system of claim 8, wherein the battery management unit isfurther configured to, after the battery management unit powers down,subsequently power up again after a period of time to determine whetherthe battery pack is operatively coupled with the battery charger. 10.The power tool system of claim 8, wherein, when the battery managementunit determines that the temperature is not in an operational range, thebattery management unit prevents use of the battery.
 11. The power toolsystem of claim 9, wherein, when the battery management unit determinesthat the temperature is not in an operational range, the batterymanagement unit prevents use of the battery